1. Field of the Invention
The present invention relates to solid-electrolyte fuel cells.
2. Description of the Related Art
A solid-electrolyte fuel cell directly converts chemical potential contained in fuel into electric energy by an electrochemical means.
FIG. 1 is a sectional view of a cell stack of a solid-electrolyte fuel cell in which, for example, three cells are stacked.
A cell 1a is a minimum unit for generating electricity by the reaction of the solid-electrolyte fuel and includes a three-layered film 2a as a generating section and a pair of separators 3a and 3b which sandwich the three-layered film 2a from both sides. The three-layered film 2a includes an air electrode 4a, a solid-electrode film 5a, and a fuel electrode 6a. A stack including a plurality of cells 1a is referred to as a cell stack 7a.
The solid-electrode film 5a is rectangular, and as its material, for example, yttria-stabilized-zirconia (YSZ) is used. As a material for the air electrode 4a, for example, lanthanum manganite (LaMnO.sub.3) is used and as a material for the fuel electrode 6a, for example, a cermet including nickel (Ni) and yttria-stabilized-zirconia (YSZ) is used.
For the separators 3a and 3b, for example, lanthanum chromite (LaCrO.sub.3) is used. Air is fed into the air electrode 4a of the three-layered film 2a through a groove 8a functioning as a gas channel, and fuel gas is fed into the fuel electrode 6a through a groove 8b functioning as a gas channel. The adjacent cells 1a are electrically connected to each other. In the cell 1a shown in FIG. 1, air and fuel gas flow on either surface of the three-layered film 2a.
FIG. 2 shows another example of a solid-electrolyte fuel cell. In FIG. 2, a separator 3c is joined to the side of an air electrode (not shown in the drawing) of a three-layered film 2b which includes the air electrode, a solid-electrolyte film, and a fuel electrode to constitute a cell 1b. A plurality of the cells 1b are stacked, and a conductive felt layer 14 provided with a fuel gas channel is placed between a fuel cell (not shown in the drawing) of one cell 1b and a separator 3c of another cell 1b lying on the fuel cell. In such a cell, air is fed into the separator 3c which is joined to the air electrode of the three-layered film 2b, and fuel gas is fed into the conductive felt layer 14 on the side of the fuel cell. The fuel electrode of the cell 1b and the separator 3c lying on the fuel electrode are electrically connected by the conductive felt layer 14, and thus, a cell stack 7b is constituted. A sealant composed of glass or a composite material, for example, including ceramic and glass is used for the peripheries of the separator 3c and for the outer sides of the conductive felt layer 14 along the fuel gas channel. The sealant also prevents the fuel gas from leaking out of the fuel cell.
An advantage of such a flat-type solid-electrolyte fuel cell is a high output per unit volume. By decreasing the thickness of the three-layered film, the number of cells per unit thickness can be increased, and also since electric current flows perpendicular to the cell plane, the internal resistance can be reduced.
The structure of the solid-electrolyte fuel cell described above requires that no leakage of gas occur where the three-layered film and the separator are joined to each other so that the air and fuel gas fed do not mix. For the purpose of sealing gas, a glass bonding agent or a glass-ceramic composite bonding agent is generally used.
With respect to these bonding agents, however, since glass softens at the operating temperature (800.degree. C.-1,000.degree. C.) of the solid-electrolyte fuel cell, the bonding strength between the three-layered film and the separator weakens. Thus, during the operation or the temperature-rising period of the solid-electrolyte fuel cell, the three-layered film and the end of the separator separate and the ability to seal gas decreases.
For the forgoing reasons, there is a need for a solid-electrolyte fuel cell which has adhesiveness to prevent the three-layered film and the end of the separator from separating and which has the ability to seal gas during the operation or the temperature-rising period of the solid-electrolyte fuel cell.